A 100 ohm unshielded twisted cable is a conventional medium for Ethernet transmission over copper cables. Standard-dictated requirements for the transmitter and the receiver at the two ends of the cable in terms of return loss may translate into a well-controlled and matched impedance of the transmitter and receiver circuitry.
Different standards may involve managing different line voltages as well as different transmitter amplitudes. The transmitter and the receiver are matched to the 100 ohm impedance of the line.